The present invention relates to a digital fluid level sensing probe system. A specific application involves liquid level sensing in a hostile environment such as the level of hot oil subjected to engine vibration in an automotive or other vehicle. State of the art in the case of automotive vehicles is limited to provision of a low level signal visible on the dashboard with no intermediate level information available other than by raising the hood and reading the level on a dip stick. This typically involves removing the dip stick, wiping it off, reinserting it, removing it for reading and reinserting it. The hostile environment precludes conventional liquid level sensing systems such as employed for fuel gages and has called for a unique solution to measuring fluid levels, preferably to meet the following conditions:
(1) High temperature, high shock/vibration, volatile fluid, presence of contaminents.
(2) Low cost, mass producible, robust, repeatability and product uniformity.
(3) Universally applicable in any shape container, although specifically designed to accommodate the wide-short profile of the automobile oil pan.
(4) Small probe profile in terms of volume displacement, especially in probe diameter.
(5) High resolution of volume measured per reading.
(6) Completely digital probe and electronics with data output directly in Binary-Coded-Decimal format.
(7) Low power consumption.
The closest prior art, of which applicants are aware, to the system disclosed herein comprise three U.S. patents found in a preliminary search of the art.
Elis U.S. Pat. No. 3,935,739 discloses a capacitive probe with one common electrode and individual opposite electrodes. An AC signal is impressed on the probe and simultaneously to all capacitive elements. The presence of dielectric material yields an analog current at the individual capacitor which is greater than the current yield in the absence of dielectric material. The AC current(s) are rectified and either summed to drive an analog meter, or shaped and synchronized to drive a digital display. Inherent limitations result from the following characteristics: the system measures absolute signal strength and is therefore subject to variations from temperature, Contaminants, and dielectric constants; nonuniformity of capacitance values among the individual segments may cause inaccuracies; relatively large size requirements limit applications involving small containers.
Johnston U.S. Pat. No. 3,343,415 discloses a cylindrical capacitor comprising a common electrode (inner) with individual electrodes (outer) spaced along a vertical (longitudinal) axis. Thee detection methodology compares the signal output from any two adjacent capacitors. If both capacitors are immersed or if both capacitors are above the liquid level, the output signals are the same. If one capacitor is immersed and the other is above the liquid level, the outputs differ by the effect of the dielectric constant of the liquid. Each capacitor is assigned a unique level and therefore the level of the fluid can be determined by the detection of a difference signal. As in the Elis reference, nonuniformity of capacitance values among the individual segements may cause inaccuracies and relatively large size requirements limit applications as to container size. The system uses a differential comparator method but the electronic implementation may involve inaccuracies due to isolating devices in series with the data signal.
Johnston U.S. Pat. No. 3,552,209 discloses a specific application of the capacitive probe employed in the Johnston U.S. Pat. No. 3,343,415, which deals with a condition for measuring dynamic levels such as ocean swells and tidal effects. It involves the use of sampling techniques whereby liquid levels are instantaneously converted to pulse trains where the number of pulses in the train is a function of the liquid level and the time interval over which the sample is taken. The pulse count is averaged over a period of time and the mean level is determined. This system eliminates anomolies (infrequent disturbances) by averaging their effects.
.Iadd.Pope U.S. Pat. No. 4,589,077 discloses a multi-segment capacitive probe that includes a column of capacitors extending through a liquid level interface. Each capacitor of the probe is sequentially measured in response to microprocessor controlled instructions. A common electrode is employed serving as one half of all probe capacitors. Similar techniques are also disclosed in Bristol U.S. Pat. No. 4,295,370 and Matsumara et al U.S. Pat. No. 4,434,657. .Iaddend.